1. Field of the Invention
The invention relates to the field of the use of magnetic resonance in medicine to measure iron stores in tissue, and in particular to quantitatively and specifically measure in vivo ferritin and closely related substances in tissue.
2. Description of the Prior Art
During the past decade, a number of studies have implicated iron as a central culprit in various diseases including some cancers. A genetic disease, hemochromatosis, which causes excessive accumulations of iron in tissue and which can be fatal, is estimated to occur or to be at risk in an estimated 1.4 million Americans. The accumulation of iron stores in tissue has also been implicated in various studies in liver damage, arthritis, diabetes, impotence, heart failure and various neurological disorders such as Alzheimer's and Parkinson's disease. Iron stores is understood to mean ferritin or ferritin like proteins, which is the biological form for storage of iron.
The deleterious effects of excessive iron stores levels in tissue is thought to be related to its ability to catalyze the production of hydroxil radicals from other free radicals which naturally occur within the human body as well as direct toxic effects of iron itself. The largest part of iron stores in human tissue normally occurs in a complex protein called ferritin, the storage form of iron. Free radicals such as superperoxides, as well as other substances which occur in the body can remove the iron from ferritin, where it is harmless, to catalyze the formation of more destructive radicals which are believed to be linked to the above disease states.
However, one of the major stumbling blocks in attaining a clear idea how ferritin or other iron stores molecules relate to disease states has been the inability to quantitatively and specifically measure ferritin in vivo. Prior art biochemical assays in many cases can only be conducted postmortem or through biopsy. All biopsies, but especially brain biopsies are subject to risk, expensive, and traumatic to the patient.
The noninvasive use of magnetic resonance imaging to measure iron stores is well known, but remains controversial and is by no means universally accepted or considered well established. The nuclear magnetic resonance of hydrogen atoms bound in water in biological tissue can be directly measured. The radio frequency signal produced by relaxational hydrogen atoms in a nuclear magnetic resonance experiment will decay with two characteristic decay rates or periods T.sub.1 or T.sub.2. The T.sub.1 decay is based upon spin-lattice couplings, while the T.sub.2 decay period is based upon spin-spin couplings among the protons. These decay periods are affected by the molecular environment in which the hydrogen is present. Throughout the remainder of the specification the symbols T.sub.2 will be used to refer to the T.sub.2 relaxation time. The relaxation rate corresponding to T.sub.2 is denoted by the symbol, R.sub.2. The relation between the T.sub.2 relaxation time and the R.sub.2 relaxation rate is given by R.sub.2 =1000/T.sub.2. Therefore, either parameter may be used interchangeably through this algebraic equivalence, or for that matter any uniquely related third parameter or measure may used if more practical. Currently, most nuclear magnetic resonance imaging (MRI) equipment provides a readout in terms of T.sub.2 and therefore, T.sub.2 will be used as the preferred measure. The presence of magnetic (e.g. paramagnetic, ferromagnetic, etc.) materials such as iron in tissue has been found to have a material effect upon the T.sub.2. Iron, in turn, occurs naturally throughout human tissue and in blood.
Brain extrapyramidal grey matter nuclei exhibit a lower T.sub.2 and this T.sub.2 shortening appears to be related to high iron concentrations in these grey matter nuclei, see, Drayer et. al., AJNR 7, 373 (1986); Duguid et. al., J. Ann.Neurol 20, 744 (1986); Coffey et.al., J. Neuropsych. Clin. Neurosci. 1, 400 (1989); Schaffert et. al., Neurology 39, 440 (1989) and Bizzi et. al., Radiology 177, 59 (1990). The capability of MRI to quantify iron levels in vivo remains controversial as some investigators report a lack correlation with postmortem tissue iron levels in T.sub.2 values, see, Chen et. al., Radiology 173, 521 (1989); and Brooks et. al., J. Neurol. Neurosurg., Psychiatry 52, 108 (1989). Nevertheless, multiple lines of evidence support an association between T.sub.2 shortening and tissue iron levels. First, many investigators observed T.sub.2 shortening in disorders with known abnormal iron accumulation in the brain and liver, see, Drayer, Radiology 173, 311 (1989); Duguid et. al., J. Ann. Neurol 20, 744 (1986); Coffey et. al., J. Neuropsych. Clin. Neurosci. 1, 400 (1989); Brasch et. al., Radiology 150, 767 (1984); Leung et. al., J. Comput. Assist. Tomogr. 8, 446 (1984); Stark et. al., Radiology 154, 137 (1985); Gomori et. al., J. Comput. Assist. Tomogr. 9, 972 (1985); Gomori et. al., Radiology 157, 87 (1985); Johnston et. al., Am. J. Med. 87, 40 (1989); and Thulborn et. al., AJNR 154, 291 (1990). Second, some postmortem studies report that T.sub.2 shortening corresponded to increased iron levels, see, Duguid et. al., J. Ann. Neurol 20, 744 (1986); Coffey et. al., J. Neuropsych. Clin. Neurosci. 1, 400 (1989); and Schaffert et. al., Neurology 39, 440 (1989). Third, age-related increase in brain iron in normal humans has also been demonstrated in vivo using magnetic resonance techniques, see, Aoki et. al., Radiology 172, 381 (1989).
The extrapyramidal system contains the highest concentration of iron in the brain; signal levels between one-and-a-half to almost two times as high as that in liver, see, Hallgren et. al., J. Neurochemistry 3, 41 (1958). The largest single fraction of tissue iron is stored in the iron storage protein, ferritin, see, Hallgren et. al., J. Neurochemistry 3, 41 (1958); and Hill et. al., Brain Iron: Neurochemical and Behavioral Aspects, chapter 1, Taylor and Francis (1988).. Ferritin molecules are comprised of a multisubunit protein shell surrounded by crystalline core of hydrous ferric oxide that may include up to as many as 4500 ferric iron atoms. The association between high iron levels and central nervous system damage has been observed in a variety of disorders. Involvement of iron in the process of lipid peroxidation has been suggested as a common mechanism for such damage, see, Park et. al., Neurology 25, 1172 (1975); Sadeh et. al., Ann. Neurol. 7, 286 (1980); and Kim et. al., Neurology 31, 774 (1981). Therefore, methods that can quantify specific physiological iron compounds, such as ferritin in vivo, could be clinical value in disorders involving brain extrapyramidal nuclei, Duguid et. al., J. Ann. Neurol 20, 744 (1986); Coffey et. al., J. Neuropsych. Clin. Neurosci. 1, 400 (1989); Schaffert et. al., Neurology 39, 440 (1989) and other tissues.
It is known that ferritin has a strong magnetic effect that results in marked T.sub.2 shortening in vitro and in vivo. Therefore, nuclear magnetic resonance relaxation times have been used to visualize evolution of hemorrhages, Gomori et. al., J. Comput. Assist. Tomogr. 11, 684-690 (1987).
It is known that the enhancement of iron-related contrast as seen in magnetic resonance images is dependent on field strengths, but it has not been previously known that the field dependence could be used in any way to be specific to ferritin or that it was a quantitative measure of the ferritin that would have any clinical utility, see, Schenck et. al., Book of Abstracts, Volume 1, Society of Magnetic Resonance in Medicine (1989).
The invention is directed to a method which utilizes the field dependence of ferritin induced T.sub.2 shortening as a way of specifically identifying and quantifying iron stores levels. What is needed, therefore, is a methodology whereby specific identification and quantitative in vivo measurements of patients can be made reliably, and wherein measurements of ferritin and closely related iron containing proteins or substances can be selectively or specifically made.